1. Technical Field
The present invention relates to an ND filter. The ND (neutral density) filter is used for stopping a part of incident light so as to decrease the amount of transmitting light uniformly over an entire visible range.
2. Related Art
Generally, when the brightness of an object is too high for an imaging system such as a camera and a video, even if an aperture stop is minimized (namely, an aperture diameter is minimized), the aperture stop may still allow an excessive amount of incident light to enter into a light-sensitive surface. To solve this problem, it is a common practice to attach an ND filter to a part of the imaging system so as to limit the amount of light incident on the light-sensitive surface. In this case, the ND filter has flat spectral characteristics of simply decreasing the amount of incident light, and therefore needs to ensure uniform transmittance over an entire visible range of wavelength. Until now, a plastic film-based ND filter has been used for the imaging system such as a camera and a video in order to uniformly decrease the amount of light over the entire visible range.
In recent years, there have been used thin-film laminated ND filters excellent in optical characteristics and durability as disclosed in Patent Documents 1 through 3.
[Patent Document 1] Japanese Patent Laid-Open Publication No. Sho 52-113236
[Patent Document 2] Japanese Patent Laid-Open Publication No. Hei 07-063915
[Patent Document 3] Japanese Patent Laid-Open Publication No. 2003-043211
Patent document 1 discloses the ND filter composed of alternate layers of a metal thin film (Ti, Ni, and the like) and a dielectric film (MgF2). That is, according to patent document 1, the metal film is used as a light-absorbing film.
Patent document 2 discloses an ND filter composed of alternate layers of two types or more of Ti metal-oxide films (with extinction coefficient k ranging from 1.0 to 3.0) and dielectric films (Al2O3, SiO2, and MgF2). Patent document 2 uses lower oxides of Ti (TiO, Ti2O3, Ti3O5, Ti4O7, and the like) as starting materials for the absorbing films composed of two types or more of Ti metal-oxide films.
Patent document 3 discloses a thin-film ND filter composed of a light-absorbing film and a dielectric film laminated on a transparent substrate such as a plastic film. The light-absorbing film is made of a metal material and is formed by evaporation. The light-absorbing film is formed by introducing oxygenic mixed gas during the deposition, and contains a metal oxide generated by maintaining the degree of vacuum.
As described in any of patent documents 1 through 3, the thin-film laminated ND filter is composed of a metal light-absorbing film and an inorganic insulation dielectric film laminated on a transparent substrate such as a plastic film. Accordingly, the outmost surface layer of the lamination is formed by either of the metal light-absorbing film or the inorganic-insulation dielectric film. When the metal light-absorbing film is used for the outmost layer, however, the outmost layer is easily flawed and therefore becomes less abrasion-resistant. When the inorganic insulation dielectric film is used for the outmost layer, by contrast, the outmost layer more excels in the abrasion resistance but is easily electro-statically charged. The static electricity collects dust, causing the yield rate of the product to decrease. That is, using the dielectric film for the outmost layer degrades the anti-electrostatic property.